In most of such conveyor installations, a secondary belt-cleaning function is performed as the belt leaves the head pulley in its return run. The secondary cleaning function is performed by a trailing-arm scraper which is pivotally mounted below the head pulley and has arms extending generally parallel to the return run for mounting scraper blades which are biased against the return run of the belt to remove debris which is not removed in the primary discharge operation on the head pulley. The trailing-arm arrangement for the secondary scraper permits splices or other obstructions projecting from the conveyor belt to displace the scraper away from the belt as the belt carries the obstructions into the path of movement of the blade. The trailing-arm scrapers are conventionally segmented, with each segment separately biased so that displacement of one segment may occur without losing the cleaning effect of the other segments.
In many prior scraper assemblies, the arm and blade were designed to allow pivotal movement or twisting of the blade about an axis perpendicular to the blade surface and parallel to the line of travel of the conveyor belt so that pivotal movement of the blade would cant the scraping edge for the apparent purpose of allowing the scraping edge to follow tilting movement of the moving belt. It has been found, however, that obstructions may also cant scraping edge of the belt, which is likely to detract from the operation of the scraping assembly and may cause the corners of the scraping edge to dig into the belt and cause premature wear.
My co-pending application, Ser. No. 07/457,112 filed Dec. 26, 1989, discloses an improved mounting arrangement for trailing-arm scrapers which facilitates installation removal and replacement of the scraper blades as they become worn or damaged in use. Prior to the invention, the most common mode for replacement of the worn blade was to replace the entire arm and blade assembly. In this mounting, the blade is firmly secured to the blade arm by a releasable connection, so that the blade assembly may be refurbished by mounting a replacement blade on the existing arm after it is removed from its operating position. While this mounting arrangement has proved helpful in solving the most common problems of trailing-arm scrapers, it has been found that in many installations, there are problems which arise when belt travel is reversed.
In normal conveyor installations, there are safety mechanisms to prevent belt reversal when power is removed from the driving pulleys, but in use, such safety devices become ineffective and frequently in older installations, particularly where the belt is used to convey material up an incline, when the belt is arrested, the conditions cause the belt to reverse its travel. When the belt is reversed, the trailing-arm scraper blades tend to dig into the belt and distort the belt, and when the reverse travel is severe and the belt has foreign objects projecting from it, the reverse travel of the belt causes the scraper blades to dig into the belt and damage the belt and sometimes damage the entire scraper blade assembly beyond repair.
U.S. Pat. No. 4,696,389 illustrates a mechanism which pivots the scraper arm away from the belt during reversal of the belt, and U.S. Pat. No. 4,344,525 illustrates a scraper device having shear pins which break to avoid damage to the scraper device or the belt. Other mechanisms have been provided which respond to the reversal of the belt to displace the scraper blade away from the belt when the belt is driven in the opposite direction. Such mechanisms have not been entirely satisfactory because of the modifications which must be made in the conveyor drive, or else because of the expense of the modified scraper assembly.